define my own web printing attribute?

I implemented the web printing feature by following the white paper guide, on class Z_PRINT_HELP_SERVICE_ALL_DP. Now I want to add my own fields in the web page, so I want to add additional attributes for Z_PRINT_HELP_SERVICE_ALL_DP.

Currently, the typical attributes are: P_WIDTH, P_PAGE_WIDTH, P_HEIGHT, P_PAGE_HEIGHT, etc. I tried to insert a new attribute called "P_ZZZ", but it does not work. I tried to reference P_ZZZ in program Z_PRINT_HTML_TEMPLATE, but Z_PRINT_HTML_TEMPLATE does not take it. BTW, Z_PRINT_HTML_TEMPLATE is included in class Z_PRINT_HELP_SERVICE_ALL_DP, as we can see in SE80.

I am confused. How to add my own attribute to the class? I am not an ABAPer.